MAXI J1305-704: Swift/XRT detection of dips, possible 1.5 hour period

ATel #4044; J. A. Kennea (PSU), Y. J. Yang, D. Altamirano (UvA), P. A. Evans (U Leicester), H. A. Krimm (CRESST/GSFC/USRA), P. Romano, V. Mangano (INAF-IASFPA), P. Curran (CEA-Saclay), K. Yamaoka (AGU) and H. Negoro (Nihon U.)
on 15 Apr 2012; 18:01 UT
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Credential Certification: Jamie A. Kennea (kennea@astro.psu.edu)

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We report on observations of MAXI J1305-704 (Sato et al., ATEL #4024) taken with the Swift X-ray Telescope (XRT). We have been performing regular short (1ks) observations of this object. During the observation beginning April 12th, 2012 at 17:53 UT we see a feature in the light-curve that appears to be a dip, similar to those seen in MAXI J1659-152 (e.g. Kennea et al., 2011, ApJ, 736, 22). This dip is accompanied by spectral hardening, likely due to increased photoelectric absorption during the dip.

In order to investigate these dips, we requested a long (10ks) observation of MAXI J1305-704, these data were taken on April 14th starting 14:29UT. Unfortunately as Swift is in a low-earth orbit we are not able to obtain complete coverage of the light-curve during this period, so the observations consist of 4 ~2500s observations, every 96 minute Swift orbit. A 5th orbit was also taken as part of the regular monitoring observations.

These observations reveal a large amount of variability in the MAXI J1305-704 light-curve, including one large dipping event, and several other smaller dips. 3 out of 5 orbits show dips with associated spectral hardening, and all orbits show evidence of dips. The 3rd orbit (17:51-18:35UT) shows the longest, deepest dipping episode with spectral hardening, with the main dip episode lasting approximately 500 seconds, and being preceded by a shorter (~100s long) dip approximately 500 seconds before the main dip. During the main dip the count rate drops from ~60 c/s to a minimum of ~15 c/s. The noisy and uneven nature of the dip would suggest that these are not eclipses.

The irregular nature of the dips makes determining at exact periodicity with these data difficult, however based on the recurrence of the dipping episodes in all 5 orbits, we suggest a preliminary periodicity of MAXI J1305-704 of ~1.5 hours. We note however, that this periodicity is close to the Swift 96 minute orbital period, so more observations over a longer baseline, or with an instrument capable of continuous coverage of the light-curve, are required to better pin down this period. Utilizing a Lomb-Scargle Periodogram, the strongest peak is at 2.7 hours, although better quality data are needed for this to be a firm detection, we cannot rule out this longer periodicity.

If this is the period of MAXI J1305-704, and it is a black-hole LMXB, this would make it the shortest period black-hole LMXB yet known, the previous record holder being MAXI J1659-152 at ~2.4 hours (e.g. Kuulkers et al., ATEL #2912). However, we note that shorter periods have been seen in Neutron Star LMXBs, and the nature of MAXI J1305-704 has yet to be definitively determined.